Insulated food serving trays and lids therefore formed of a thermoplastic shell which permanently encases an insulating material are known. The examples are U.S. Pat. Nos. 3,532,247, 3,754,640 and 3,799,386. These insulated trays and lids are generally formed by sealed together two panels to form a hollow shell. Through a small opening in the shell, a material is injected and expanded to fill the intricately shaped chamber between the two panels. A polyurethane foam has generally been used with a "foamed-in-place" process. Such "foamed-in-place" insulated trays and lids have a serious disadvantage in that the polyurethane tends to continually expand with time and exert an internal pressure on the seam area of the tray. Also, with temperature changes during hot water washing, at over 200.degree. F. the foam can exert an additional pressure on the seam area which can cause it to rupture.
It is important that the seam area between the two panels have a very high degree of structural integrity. Any minute gap or fracture can permit dishwater to enter the insulated chamber between the panels where it may be possible for bacteria to grow. Since the panels are of a thermoplastic material, they cannot be as firmly or as readily crimped and rolled as metal panels can. The thermoplastic tray and lid are much preferred over metal units because of their lighter weight and attractiveness.
In the past the thermoplastic panels of the insulated food trays and lids have been bonded together in a generally horizontal bond between two horizontal flanges about the periphery of the tray or lid. The patents mentioned above, as well as U.S. Pat. No. 3,613,933, show these horizontal flange seals. Tray, U.S. Pat. No. 3,613,933, shows the type of prior art seals. Although the lid of this patent does have a short downturn skirt to position the lid up on the tray, the primary sealing area would be in the horizontal area as in the tray of this patent.
The face to face bond between two horizontal flanges of the thermoplastic insulated trays and lids of the past have had serious disadvantages. First, the outer edges of the two flanges provided a somewhat rough and unsightly narrow or knife-like edge which was particularly exaggerated if the two flanges were not perfectly flush and smooth at their outer hinge. Secondly, the internal pressure created on the horizontal beams because of the "foamed-in-place" polyurethane insulating material tended to create a peeling action at the seam area caused by the vertical expansion of the foam.
Perhaps one of the most serious problems with the horizontal seams of the prior art thermoplastic food trays and lids has been the failure of such seams during lateral banging in the seam area when handling the trays and lids during washing, filling and stacking, etc. It should be noted that when two trays are placed on a table top their horizontal seams are aligned with each other much in the same way as bumpers on automobiles. When the trays are moved quickly along a filling track or other structure, violent banging of the trays can and does occur. The shock of such banging frequently occurs in a very narrow area of the seam, such as a curved corner or curved end. The shock causes localized distortion of the two halves and tensile failure of the weak butt joint. This is particularly a problem when combined with the internal pressure caused by "foamed-in-place" material generating a peeling action on the horizontal seam.